1. Field of the Invention
The present invention relates to a surface-treated steel sheet which is optimal for uses of automobiles, household electrical appliances and building materials. In particular, the invention relates to a surface-treated steel sheet of an environmental adaptation type which does not contain hexavalent chromium in a surface-treated film.
2. Description of the Related Art
On steel sheets for household electrical appliances, steel sheets for building materials and steel sheets for automobiles, for the purpose of enhancing corrosion resistance (for example, white rust resistance and red rust resistance), steel sheets in which a surface of a zinc based plated steel sheet or an aluminum based plated steel sheet is subjected to a chromate treatment with a treatment liquid containing, as a major component, chromic acid, bichromic acid or a salt thereof have hitherto been widely used. This chromate treatment is an economical treatment method having excellent corrosion resistance and capable of being relatively simply achieved.
The chromate treatment uses hexavalent chromium which is a substance subjective to the pollution control. But, since not only this hexavalent chromium is treated in a closed system in a treatment step, but it is possible to make the elution of chromium from a chromate film due to a sealing action by an organic film formed on an upper layer substantially zero, the human body or environment is not substantially polluted by the hexavalent chromium. However, with an increasing interest in the recent global environmental issue, not only conventional laws and regulations which attach importance to the working environment and wastewater treatment but laws and regulations which attach importance to the environmental load and environmental harmony are being started. Also, there is the historical background that manufacturers are evaluated in terms of a degree of contribution to the environment, and a move to reduce the use of heavy metals including hexavalent chromium is increasing.
Under such a background, a number of proposals regarding a technology for controlling white rust of a hexavalent chromium-free zinc based plated steel sheet (chromate-free technology) have been made. For example, Patent Documents 1 and 2 propose a surface treatment agent containing Al, a phosphorus compound, silica and an aqueous organic resin emulsion and a metal material applied therewith. Also, Patent Document 3 proposes a zinc based plated steel sheet prepared by applying and drying a mixed aqueous solution of a primary phosphate of a polyvalent metal and a metal oxide sol to form an amorphous film and then forming an organic coat layer. Furthermore, Patent Documents 4 and 5 propose a steel sheet prepared by forming, as a lower layer, a composite oxide film layer containing an oxide fine particle, phosphoric acid and/or a phosphoric acid compound and at least one metal selected from Mg, Mn and Al and forming, as an upper layer thereof, an organic film.                Patent Document 1: JP-A-11-350157        Patent Document 2: JP-A-2000-26980        Patent Document 3: JP-A-2000-129460        Patent Document 4: JP-A-2002-53979        Patent Document 5: JP-A-2002-53980        
In uses to which a zinc based plated steel sheet is applied, parts which are heated in a temperature region of the melting point of zinc or higher (from about 500 to 600° C.) are present not a little. For example, in a heat exchanger in an outdoor unit of an air conditioner, in brazing of a copper pipe and an aluminum-made evaporator, for the purpose of preventing melting of aluminum caused due to heating using a gas burner, a zinc based plated steel sheet is disposed between the copper pipe and the evaporator such that a flame of the burner does not come into direct contact with aluminum. When the foregoing related-art surface-treated steel sheets are applied to such a use, since the film is composed mainly of an organic resin, it is discolored yellow or brown due to heat decomposition, whereby the appearance becomes poor. For that reason, it is substantially impossible to apply the related-art surface-treated steel sheets.
In order to solve such a problem, a chromate-free technology which is excellent in heat discoloration resistance is proposed. For example, Patent Documents 6 and 7 propose an inorganic-rich film containing, as major components, a primary phosphate and colloidal silica. Also, Patent Document 8 proposes a double-layered film in which an inorganic-rich film containing, as major components, a primary phosphate and colloidal silica is disposed as a lower layer and a silicate film and/or a silicon resin is disposed as an upper layer.                Patent Document 6: JP-A-2000-79370        Patent Document 7: JP-A-2001-348672        Patent Document 8: JP-A-2004-91826        
However, the inorganic-rich films of Patent Documents 6 and 7 are extremely low on a level of corrosion resistance and are difficult for application as a replacement of the chromate film. On the other hand, though the double-layered film of Patent Document 8 is on a level of corrosion resistance on which it is applicable as a replacement of the chromate film, an expensive silica or silicon resin is used, and therefore, it is problematic from the standpoint of costs. Also, since the films which are brought by these technologies are easy to cause a phenomenon where the film is discolored black under a humid environment (blackening), they are difficult for application in a heat exchanger where dew condensation is easy to occur and are restricted with respect to the storage environment during transportation of products, and therefore, they are not practical useful.